Direct imaging can be executed by using a series of lenses or apertures to focus an image on a plane and then measuring or recording the local intensity or power of the radiation on that plane. This method of direct imaging, however, suffers from limited resolution owing to the Rayleigh Criterion.
For example, the Rayleigh Criterion has led to a plethora of large diameter observatories in the quest of higher and higher resolution astronomy. Far-field linear optics can achieve sub-Rayleigh resolution and the long-held Rayleigh Criterion can be a mere limitation of the direct imaging technique.
For example, in direct imaging, photon counting on a 2D plane can be executed in a localized position array of detectors such as in a charge coupled device (CCD). This technique, however, neglects the wave nature of the image photons, treating them purely in a particle capacity. Thus, the phase information of the image may not be acquired.
In direct imaging, the finite dimensions of the optical aperture, D, induces diffraction patterns which limit the angular resolving power of the system from two separate distant physical objects to θ=1.22λ/D. If photon counting can be executed on mode by mode basis instead of in a position basis, then the resolution of the system may become quantum limited.